Spanners with strain gauges are more and more popular because it can measure the strains in operation so that the user can determine the applied force and to control the size of the applied force not to be too large so as to destroy the element to be released.
There are many kinds of spanners with strain gauges are developed. In one prior art, disclosed in Taiwan Patent No. 284306, the strain gauge spanner is installed with a strain gauge for determining the strain gauge in operation so that the user can determine that the element to be released has been released to a desired level. Thus, the element will not be too loose or too tight. In this prior art the spanner has a driving head and a rod body. The rod body has an inclined surface which is in contact with the strain gauge. When the driving head serves to drive an element, the twisting force will be transferred to the strain gauge through the inclined surface. Thus the strain gauge can measure the twisting force. If the twisting force is over a predetermined value, the strain gauge is possibly destroyed. Thus the use of the strain gauge spanner is limited.
Furthermore, in this prior art, the use of the spanner must be adjusted to an orientation matched to the inclined surface. If the user drives an element along another direction, the strain gauge cannot measure the twisting force. Thereby the operation is inconvenient. Thus, it is not practical.